What makes it easy to identify a species of birds in the wood?

This question comes from a comprehension passage about birdwatching and bird identification. The passage contrasts how beginners (children) notice birds primarily by sight with how experienced observers identify species in environments where visibility is limited.

In a wood, dense foliage and tree cover make it difficult to spot birds visually. Colour, beak shape, and feeding behaviour are hard to observe when the bird is hidden among leaves and branches. Under these conditions, the most reliable way to identify a species is by its characteristic call. Each bird species produces distinctive songs and calls, and with practice, an observer can learn to recognise which species is present simply by listening.

The key phrase is "with practice" - learning to identify birds by their calls requires time and repeated exposure, but once mastered, it becomes the most effective method for identifying species in wooded habitats where direct sighting is difficult.

The correct answer is their characteristic calls with practice.